1. Field of the Invention
This invention relates to a bi-polar separator plate for use in a proton exchange membrane fuel cell stack. The separator plate is hydrophilic and has a controlled porosity that facilitates in the internal humidification of the fuel cell as well as the removal of product water from the fuel cell, all the while providing means for controlling the temperature of the fuel cell stack.
Generally, fuel cell electrical output units are comprised of a stacked multiplicity of individual cells separated by bi-polar electronically conductive separator plates. Individual cells are sandwiched together and secured into a single staged unit to achieve desired fuel cell energy output. Each individual cell generally includes an anode and cathode electrode, a common electrolyte, and a fuel and oxidant gas source. Both fuel and oxidant gases are introduced through manifolds, either internal or external to the fuel cell stack, to the respective reactant chambers between the separator plate and the electrolyte.
2. Description of Prior Art
There are a number of fuel cell systems currently in existence and/or under development which are designed for use in a variety of applications including power generation, automobiles, and other applications where environmental pollution is to be avoided. These include molten carbonate fuel cells, solid oxide fuel cells, phosphoric acid fuel cells, and proton exchange membrane fuel cells. One issue associated with successful operation of each of these fuel cell types is the control of fuel cell temperature and the removal of products generated by the electrochemical reactions from within the fuel cell.
Commercially viable fuel cell stacks may contain up to about 600 individual fuel cell units, each having a planar area up to 12 square feet. In stacking such individual cells, separator plates separate the individual cells, with fuel and oxidant each being introduced between a set of separator plates, the fuel being introduced between one face of the separator plate and the anode side of an electrolyte and oxidant being introduced between the other face of the separator plate and the cathode side of a second electrolyte. Cell stacks containing 600 cells can be up to 20 feet tall, presenting serious problems with respect to maintaining cell integrity during heat up and operation of the fuel cell stack. Due to thermal gradients between cell assembly and cell operating conditions, differential thermal expansions, and the necessary strength of materials required for the various components, close tolerances and very difficult engineering problems are presented. In this regard, cell temperature control is highly significant and, if it is not accomplished with a minimum temperature gradient, uniform current density will not be maintainable, and degradation of the cell will occur.
In a proton exchange membrane (PEM) fuel cell, the electrolyte is an organic polymer in the form of a proton conducting membrane, such as a perfluorosulfonic acid polymer. This type of fuel cell operates best when the electrolyte membrane is kept moist with water because the membrane will not operate efficiently when it is dry. During operation of the cell, water is dragged through the membrane from the anode side to the cathode side along with proton movement through the membrane. This tends to dry the anode side of the membrane, and also tends to create a water film on the cathode side of the membrane. The cathode surface is further wetted by product water which is formed in the electrochemical reaction. Thus, it is critical to the operation of the PEM fuel cell that the product water be continuously removed from the cathode side of the membrane while maintaining the anode side of the membrane wet to facilitate the electrochemical reaction and membrane conductivity.
The issue of water management in a proton exchange membrane fuel cell is addressed by a number of U.S. patents. U.S. Pat. No. 4,769,297 teaches the use of a solid polymer fuel cell in which water is supplied with the anode gas to the anode side of the membrane. Some of the water migrates through the stack from cell to cell, water migration being the result of water being dragged from the anode through the membrane to the cathode and by the use of a hydrophilic porous separator plate interposed between adjacent cell units. Water is forced through the porous separator plate by means of a reactant pressure differential maintained between the cathode and anode. The anode support plates provide a large surface area from which water is evaporated to perform the cooling function. The separator plate is indicated to be made of graphite.
U.S. Pat. No. 4,824,741 teaches a fuel cell system using a porous graphite anode plate. Water supplied to the porous plate and the anode reactant gas is humidified by evaporation from the surface of the plate. The proton exchange membrane is moistened by contact with the wet porous anode plate. A non-porous gas impervious separator plate adjacent to the cathode plate is used to prevent gas crossover from the anode to the cathode. See also U.S. Pat. No. 4,826,741; U.S. Pat. No. 4,826,742; U.S. Pat. No. 5,503,944; and PCT Application No. WO 94/15377.
Bi-polar separator plates for use in proton exchange membrane cells constructed of graphite or resin-bonded graphite carbon composite materials and having gas flow channels are taught by U.S. Pat. No. 4,175,165. This patent also teaches the treatment of the bi-polar separator plates by coating the surfaces with a wetting agent, such as colloidal silica sols, to render the surfaces thereof hydrophilic. In this way, water generated in the fuel cell is attracted away from the electrodes for subsequent disposition. However, coating the surfaces with a wetting agent undesirably increases the electrical resistance across the plate, resulting in reduced conductivity. U.S. Pat. No. 3,634,569 teaches a method of producing dense graphite plates from a mixture of powder graphite and a thermosetting resin for use in acid fuel cells. The method employs a mixture, by weight, of 5% to 25% thermosetting phenolic resin binder and 75% to 95% sized powdered graphite. Graphite and resin bi-polar plates are also taught by U.S. Pat. No. 4,339,322 (a bi-polar plate comprised of molded thermoplastic fluoropolymer, graphite and carbon fibers), U.S. Pat. No. 4,738,872 (separator plates comprising 50 weight percent graphite and 50 weight percent thermoset phenolic resin), U.S. Pat. No. 5,108,849 (serpentine flow panels in a fuel cell separator plate composed of non-porous graphite or other corrosion resistant metal powders and thermoplastic resin, such as polyvinylidene fluoride, in a composition of 10-30 weight percent resin and 70-90 weight percent graphite powder), U.S. Pat. No. 4,670,300 (a fuel cell plate comprising 20% to 80% graphite with the balance being cellulose fibers or cellulose fibers and thermosetting resin in equal proportions), U.S. Pat. No. 4,592,968 (separator plate comprised of graphite, coke and carbonizable thermosetting phenolic resin which are then graphitized at 2650.degree. C.), U.S. Pat. No. 4,737,421 (fuel cell plate from carbon or graphite in the range of 5% to 45%, thermosetting resin in the range of 40% to 80%, with the balance being cellulose fibers), U.S. Pat. No. 4,627,944 (fuel cell plate from carbon or graphite, thermosetting resin and cellulose fibers), U.S. Pat. No. 4,652,502 (fuel cell plate made from 50% graphite and 50% thermosetting resin), U.S. Pat. No. 4,301,222 (separator plate made from a mixture of 40% to 65% graphite and 35% to 55% resin), and U.S. Pat. No. 4,360,485 (separator plate made from a mixture of 45% to 65% graphite and 35% to 55% resin).
We have found that there are numerous characteristics for a bi-polar separator plate for use in proton exchange membrane fuel cells which are important from a manufacturing, as well as an operational, perspective which are not addressed by the prior art. These include water permeability of the plate relative to electronic conductivity of the plate; crush strength of the plate; functionality of the plate with respect to its ability to maintain its water absorption capabilities; and the ability of the plate to undergo thermocycling between frozen and thawed conditions as are likely to be encountered in, for example, an automobile application of the fuel cell. In addition, the separator plate should be constructable from inexpensive starting materials, materials that are easily formed into any plate configuration, preferably using a one-step molding process, materials that are corrosion resistant in low temperature fuel cells and that do not require further processing such as high temperature thermal treatments, and utilizing a method for producing the plates in which the hydrophilicity and porosity of the plate can be controlled.